I'm fairly far out from Boston/Cambridge but I'm pretty much the same situation. Going in for a commute (or 9-5 event), the commuter rail is pretty good; I'm a 7 minute drive to the station. But it's basically unworkable for an evening event (or a day into evening event). Trains are maybe every 90 minutes outside of commuting hours and they're largely empty. I end up suffering the drive in, paying for parking as needed (which isn't an issue if I'm going in for my usual theater), and then a pretty easy drive home. Wouldn't even think about taking rail in for the weekend.